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The substitution provides the antiderivative $v$ needed for the integration by parts formula $\int u dv = uv - \int v du$. Without calculating $v$ via substitution, the integration by parts step cannot be completed.

Conditions: The integral requires integration by parts.; One part of the integrand ($dv$) requires substitution to integrate.; The other part ($u$) is easily differentiated.

The substitution provides the antiderivative $v$ needed for the integration by parts formula $\int u dv = uv - \int v du$. Without calculating $v$ via substitution, the integration by parts step cannot be completed.

Conditions: The integral requires integration by parts.; One part of the integrand ($dv$) requires substitution to integrate.; The other part ($u$) is easily differentiated.